With the rapid development of information technology and an increment in the working frequency of integrated circuits, the demand for low voltage high current power supplies keeps increasing in order to reduce the power consumption of circuits, and as a result, the generated voltage will be decreased to below 1.0 Volts (V). When the output voltage of a power supply decreases, the forward voltage of a diode will increase. For example, the forward voltage of a Fast Recovery Diode (FRD) or a Super-fast Recovery Diode (SRD) could be up to 1.0V to 2.0V. The increment could be approximately 0.6V even for a Schottky diode. As a result, the efficiency of the power supply will be lower. With the development of the synchronous rectifier and corresponding control technology, the application of synchronous rectifying technology is expanding quickly as it helps improve the efficiency, thermal performance, power density, manufacturability and reliability of power supplies.
A current drive synchronous rectifier obtains a drive signal by sampling the current through the synchronous rectifier. The drive signal will be generated when detecting current through the diode of the synchronous rectifier, whereas, the synchronous rectifier will turn off when the current reaches zero, and as a result, the current cannot flow from the drain to the source of the synchronous rectifier. In this way, like a diode, a synchronous rectifier features unilateral conduction, and can be used in various circuitry topologies in power supplying converters. Therefore, current drive synchronous rectifiers have a very bright future. However, prior art current drive synchronous rectifying technology has many drawbacks such as substantial power consumption, complicated circuitry structure, low working frequency, being not easy to control etc., all of which hinder its application.
As shown in prior art FIG. 1, in a current controlled synchronous rectifying drive circuit with energy feedback (such as is illustrated in U.S. Pat. No. 6,597,587), a high frequency current transducer is required to sense the current in the secondary winding of the transformer, and two current transducers are required because full-wave rectification is employed here in the center-tapped secondary winding of the transformer. Moreover, in low voltage high current applications, the high frequency high current loop of the secondary winding incorporates a current transducer, which increases the length and complexity of the current loop and cause more power consumption in the wire and lower efficiency. This not only adds difficulty in the printed circuit board (PCB) layout, but also requires thicker output wire to make the transformer, which further makes the manufacture process complicated and expensive.